This invention relates to electric resistance heating elements, and more particularly, to insulated immersion resistance heating elements having increased service life.
Electric resistance heating elements typically contain a polymeric or metal sheath which insulates a Nixe2x80x94Cr coil or wire disposed axially through the sheath. Such elements are known to experience scale growth in service, which can limit their usefulness.
Good examples of polymeric heating elements include those disclosed in Eckman et al., U.S. Pat. No. 5,586,214, issued Dec. 17, 1996; Lock et al., U.S. Pat. No. 5,521,357, issued May 28, 1996; Welsby et al., U.S. Pat. No. 4,326,121, issued Apr. 20, 1982, and J. W. Welsh, U.S. Pat. No. 3,621,566, issued Nov. 23, 1971, which are all hereby incorporated herein by reference.
Eckman et al. ""214 discloses a polymer encapsulated resistance heating element including a resistance heating member encapsulated within an integral layer of an electrically-insulating, thermally-conductive polymeric material. The disclosed heating elements are capable of generating at least about 1,000 watts for heating fluids such as water and gas.
Lock et al. ""357 discloses a heater apparatus including a resistive film formed on a substrate. The first and second electrodes are coupled to conductive leads which are electrically connected to the resistive film. The heater also includes an over molded body made of an insulating material, such as a plastic. Lock et al. ""357 further disclose that their resistive film can be applied to a substrate, such as a printed circuit board material.
Welsby et al. ""121 discloses an electric immersion heater having a planar construction which contains an electrical resistance heating wire shrouded within an integral layer of polymeric material, such as PFA or PTFE, which is wound around end portions of a rectangular frame. The frame and wound resistance wire is then secured in spaced relationship with one or more wrapped frame members, and then further protected by polymeric cover plates which allow for the free flow of fluid through the heater.
J. W. Welsh ""566 discloses a single planar resistance member having a dipped coating of thermoplastic material, such as PTFE, nylon or KEL-F, a 3M product. Welsh teaches that his element can be self-cleaning, since the heated wire is free to expand within the insulation, which is flexible.
The problems associated with metal and polymeric sheathed elements in immersed fluids are generally known. These problems are caused by the industry""s need for high watt densities. High watt densities can cause high external sheath temperatures which can damage fluid and increase scale build-up, which in turn, can result in high internal heating element temperatures which limit heater life.
The formation of hard lime scale on container walls and heating elements can be traced to the calcium (Ca) or calcium carbonate (CaCO3) content of the water in combination with the scarcity of nucleation centers in ordinary water. When the concentration of the calcium carbonate exceeds its solubility, solidification often begins on the surface of the heating element. Hard lime scale begins with a few starting points on the surface of the element which attach firmly to it and extend crystals which cling to one another in a dendritic crystallization mode. This process continues as further solidification of the mineral occurs, growing layer by layer over each successive formation of dendrites. See Kronenberg, xe2x80x9cMagnetic Water Treatment De-mystifiedxe2x80x9d, Green Country Environmental Associates, LLC, Jan. 19, 2000, which is hereby incorporated by reference.
Scale produced by residential water heaters operated on hard water at approximately 160xc2x0 F. consists principally of calcium and calcium carbonate. Differences in water quality at various sites do not generally exert a strong influence on scale composition. Minor metallic constituents, such as magnesium, aluminum and iron, generally comprise less than 3% of the scale composition.
There is a slight improvement in scale resistance associated with polymer sheathed fluid heating elements; however, even polymer-sheathed elements can overheat and fail due to scale build-up, and there remains a need in the heating element industry to minimize element failures due to this phenomena. Some of the challenges associated with improving scale induced overheating in polymer heating elements include (1) the low thermal conductivity of polymeric coatings which generally prevents thick polymer coatings from being used; (2) the need to use a greater surface area to keep the polymer below its heat deflection temperature, while providing for the application""s total wattage requirements; (3) the high manufacturing costs associated with larger surface area heaters, and (4) the management of mechanical and creep stresses due to the differences in the coefficient of thermal expansion between metallic and polymeric materials.
The present invention provides methods of heating larger quantities of liquid within storage containers, such as heating at least one gallon of water in a residential or commercial environment. The storage container is provided with a substantially renewable supply of water, a water inlet and a water outlet. The water has, in solution, a concentration of calcium, calcium carbonate, or both, which is sufficient to form a scale deposit during the heating of the quantity of water. The storage container also includes an electrical resistance immersion heating element, which further includes a resistance heating material disposed within an electrically insulating, substantially water impervious sheath. The immersion heating element has an active element portion having a watt density of no greater than about 60 watts/square inch (xe2x80x9cW/in2xe2x80x9d), preferably about 10-40 W/in2, but also has an overall wattage rating of at least about 1,000 watts (xe2x80x9cWxe2x80x9d), preferably about 2,500-4,500 W. In this first method embodiment, the immersion heating element is electrically activated to heat the water above ambient temperature sufficiently to begin the formation of a scale deposit.
The design of the immersion heating elements in this invention substantially reduces the growth of scale in the storage container, on the element surface, or both, which consequently, also increases the life of the immersion heating element. The methods and devices of this invention employ a lower heat flux or watt density to heat fluids, which yields slower scale growth. Since calcium and calcium carbonate have a decreased solubility with increasing water temperature, reducing the watt density of the element tends to slow the growth of scale. While this is not surprising, the small active element volume (xe2x80x9cenvelopexe2x80x9d) and total wattage rating requirements of the water heater industry have formerly limited element designs to high watt density, low surface area immersion heater constructions.
While it has recently been believed that higher heat fluxes tend to xe2x80x9cpopxe2x80x9d off scale, this phenomena does not reliably eliminate scale from all areas of immersion heaters. Discontinuities of heat flux within existing immersion heating elements has been known to cause scale to hang onto cooler areas and grow dendritically from the cooler areas to the heated areas.
Scale crystals will also circulate in the water bath settling on unheated surfaces as well. Scale generation, whether it be on a metal or plastic sheath, generally leads to failure of the immersion heating element, since the resistance heating material will overcompensate to maintain fluid temperature. Element failure can be detected by high leakage current, which is an indication of insulation breakdown, or an element resistance change greater than ten percent, such as when an element breaks during a burnout.
The smaller diameter, low watt density immersion heating elements of this invention have been known to generate element lifetimes greater than 1,000 hours, and even exceeding 2,000 hours at total wattage outputs exceeding 1,000 watts, without element burnout or insulation breakdown. The low watt density, small diameter water heating elements of this invention can be configured to have the same total wattage rating and xe2x80x9cenvelopexe2x80x9d size as higher watt density, larger diameter standard water heaters, yet allow for lower heater surface temperatures, lower heat flux, and slower scale growth.
The lower heater temperatures of the present immersion heaters, generate less total scale in the water tank as well. This is due, in part, to the fact that the maximum temperature that the stored water experiences in the storage container is much lower than water exposed to higher watt density heaters, even though the average bulk water temperatures are about the same. Stated differently, water in direct contact with the hotter sheath of larger diameter, higher watt density heaters, is raised to a higher temperature, and has a greater tendency to form scale, than water in direct contact with the lower temperature immersion heating elements of this invention.
It has been further discovered that, as long as water can circulate around the low watt density, small diameter heaters of this invention, i.e., the water passages are not totally blocked by scale growth, these heating elements become substantially temperature self limiting, due to further scale growth. That is, the maximum wire temperature caused by scale growth can be designed into the heater to prevent insulation breakdown or element burnout. This enables simple polymeric sheathed heating elements, or polymeric wire heaters to be employed without exceeding the melting temperature of the polymeric sheath materials, while simultaneously providing the typical power levels of about 1,000-4,500 watts required by commercial water heaters. Such total wattage ratings can be achieved within the same envelope or element volume as conventional metal sheath heaters of larger diameters, for example, those having U-shaped active element portions with diameters and watt densities of about 0.260 (200 W/in2)-0.315 (80 W/in2) inches. While providing the same envelope and total wattage as large diameter heaters, the elements of this invention provide less of a temperature gradient in the stored water, and at least 10 wt. % less scale mass, preferably at least 50 wt. % less scale mass, and as much as 96 wt. % less scale mass, in the tank.
The xe2x80x9cself-limitingxe2x80x9d temperature feature of small diameter, low watt density heaters applies equally to metal sheath-granular ceramic insulation constructions and polymer-coated wire constructions alike, although scale growth tends to collect faster and bind tighter on metal sheathing.
In conclusion, the use of a small diameter, low watt density heater is a practical solution to scale build up and shortened element life in water heating applications. The present invention is best utilized when water circulation is maintained around the active element portion as scale grows. The immersion heaters of this invention can fit within the required envelope of standard water heater element designs, which is generally limited by the 1-1.5 inch standard opening of residential hot water heaters, and the width of the standard storage containers. Typically, the envelope is approximately 1.188 in. in diameter and about 6-15 inches in length, for a total envelope of approximately 5-50 in3, preferably less than about 30 in3.
In further embodiments of this invention, a combination of a storage container and an immersion heating element is provided. The storage container is provided with a substantially renewable supply of water which contains calcium, calcium carbonate or both in a concentration sufficient to form a scale deposit during the heating of the water. The electrical resistance immersion heating element is mounted through the wall of the storage container and includes a resistance heating material disposed within an electrically insulating, substantially water impervious sheath to form an active element portion. The active element portion has a cross-sectional dimension of no greater than about 0.25 inches, preferably less than 0.125 in, and most preferably, about 0.025-0.069 in., and has a watt density of no greater than about 60 W/in2, preferably about 10-40 W/in2, and most preferably about 20-35 W/in2, while providing a total wattage rating of at least about 2500-4500 watts.
The elements of this invention can be tubular or polymer coated wire designs, or alternatively, can be disposed within, or on, the wall of the storage tank itself. One such design includes as molding a Nixe2x80x94Cr wire into a PPS tank. Such can be accomplished by blow molding the storage tank into two separate steps, with an intermediate wire wrapping step, for example.